Categorification in mathematics

introduction

general motivation for categorification
algebraic/geometric structures <-> category
we can use general properties of the category $\mathcal{C}$
It's a long established principle that an interesting way to think about numbers as the sizes of sets or dimensions of vector spaces, or better yet, the Euler characteristic of complexes.
You can't have a map between numbers, but you can have one between sets or vector spaces.
For example, Euler characteristic of topological spaces is not functorial, but homology is.
One can try to extend this idea to a bigger stage, by, say, taking a vector space, and trying to make a category by defining morphisms between its vectors.
This approach (interpreted suitably) has been a remarkable success with the representation theory of semi-simple Lie algebras (and their associated quantum groups).